This invention relates to ion thrusters and, more particularly, to the shapes of the grids used in the ion-optics system of the ion thruster.
Ion thrusters are used in spacecraft such as communications satellites for stationkeeping and other functions. An important advantage of the ion thruster over an engine using chemical propellants is that it utilizes the electrical power generated by the solar cells of the satellite to achieve the propulsion. The ion thruster has a high specific impulse, making it an efficient engine which requires very little propellant. Since the ion thruster requires relatively small amounts of the consumable propellant that is ionized, it is therefore not necessary to lift large masses of chemical fuel to orbit.
In an ion thruster, a plasma is created and confined within the body of the thruster. Ions from the plasma are electrostatically accelerated rearwardly by an ion-optics system. The reaction with the spacecraft drives it forwardly, in the opposite direction. The force produced by the ion thruster is relatively small. The ion thruster is therefore operated for a relatively long period of time to impart the required momentum to the heavy spacecraft. For some missions the ion thruster must be operable and reliable for thousands of hours of operation, and with multiple starts and stops.
The ion-optics system includes grids to which appropriate voltages are applied in order to accelerate the ions rearwardly. The grids are in a facing orientation to each other, spaced apart by relatively small clearances such as about 0.035 inches at room temperature. The grids include aligned apertures therethrough. Some of the ions accelerated by the applied voltages pass through the apertures, providing the propulsion. Others of the ions impact the grids, heating them and etching away material from the grids by physical sputtering. The heating and electrostatic forces on the grids combine to cause substantial mechanical forces at elevated temperature on the grids, which distort the grids unevenly. The uneven distortion of the grids causes adjacent grids to physically approach each other, rendering them less efficient and prone to shorting against each other. These effects are taken into account in the design of the grids and the operation of the ion thruster, so that the thruster remains functional for the required extended lifetimes. However, limitations may be placed on the operation of the ion thruster because of grid distortion, such as a relatively slow ramp-up in power during startup and operation, so that the adjacent grids do not expand so differently that they come into contact.
At the present time, the grids are usually made of molybdenum formed into a domed shape. The molybdenum resists material removal by physical sputtering. The domed shape establishes the direction of change due to thermal expansion and aids in preventing a too-close approach of the adjacent grids as a result of differences in temperatures of the adjacent grids. While the available grids are operable in current engines, it is expected that uneven expansion of the grids may limit the extension of ion thrusters to larger sizes and higher power ranges, as well as to certain desired operating ranges such as rapid start-up and acceleration.
Accordingly, there is a need for a better approach to the grids used in the ion-optics systems of ion thrusters. The present invention fulfills this need, and further provides related advantages.